# Copyright Vladimir Prus 2002.
# Copyright Rene Rivera 2006.
# Distributed under the Boost Software License, Version 1.0.
# (See accompanying file LICENSE.txt or copy at
# https://www.bfgroup.xyz/b2/LICENSE.txt)

# Supports 'abstract' targets, which are targets explicitly defined in a
# Jamfile.
#
# Abstract targets are represented by classes derived from 'abstract-target'
# class. The first abstract target is 'project-target', which is created for
# each Jamfile, and can be obtained by the 'target' rule in the Jamfile's module
# (see project.jam).
#
# Project targets keep a list of 'main-target' instances. A main target is what
# the user explicitly defines in a Jamfile. It is possible to have several
# definitions for a main target, for example to have different lists of sources
# for different platforms. So, main targets keep a list of alternatives.
#
# Each alternative is an instance of 'abstract-target'. When a main target
# subvariant is defined by some rule, that rule will decide what class to use,
# create an instance of that class and add it to the list of alternatives for
# the main target.
#
# Rules supplied by the build system will use only targets derived from the
# 'basic-target' class, which will provide some default behaviour. There will be
# different classes derived from it such as 'make-target', created by the 'make'
# rule, and 'typed-target', created by rules such as 'exe' and 'lib'.
#
#                  +--------------------------+
#                  | abstract-target          |
#                  +==========================+
#                  | name                     |
#                  | project                  |
#                  |                          |
#                  | generate(properties) = 0 |
#                  +-------------+------------+
#                                |
#                                ^
#                               / \
#                              +-+-+
#                                |
#                                |
#             +------------------+-----+-------------------------------+
#             |                        |                               |
#             |                        |                               |
# +-----------+----------+      +------+------+                +-------+------+
# | project-target       |      | main-target |                | basic-target |
# +======================+ 1  * +=============+  alternatives  +==============+
# | generate(properties) |o-----+ generate    |<>------------->| generate     |
# | main-target          |      +-------------+                | construct = 0|
# +----------------------+                                     +-------+------+
#                                                                      |
#                                                                      ^
#                                                                     / \
#                                                                    +-+-+
#                                                                      |
#                                                                      |
#          ...--+-----------------+-----------------+------------------+
#               |                 |                 |                  |
#               |                 |                 |                  |
#        ... ---+-----+   +-------+------+   +------+------+   +-------+------+
#                     |   | typed-target |   | make-target |   | stage-target |
#                     .   +==============+   +=============+   +==============+
#                     .   | construct    |   | construct   |   | construct    |
#                         +--------------+   +-------------+   +--------------+

import assert ;
import build-request ;
import "class" : new ;
import feature ;
import indirect ;
import path ;
import property ;
import property-set ;
import sequence ;
import set ;
import toolset ;


# Base class for all abstract targets.
#
class abstract-target
{
    import assert ;
    import "class" ;
    import errors ;
    import project ;

    rule __init__ ( name  # Name of the target in Jamfile.
        : project-target  # The project target to which this one belongs.
    )
    {
        # Note: it might seem that we don't need either name or project at all.
        # However, there are places where we really need it. One example is
        # error messages which should name problematic targets. Another is
        # setting correct paths for sources and generated files.

        self.name = $(name) ;
        self.project = $(project-target) ;
        self.location = [ errors.nearest-user-location ] ;
    }

    # Returns the name of this target.
    rule name ( )
    {
        return $(self.name) ;
    }

    # Returns the project for this target.
    rule project ( )
    {
        return $(self.project) ;
    }

    # Return the location where the target was declared.
    rule location ( )
    {
        return $(self.location) ;
    }

    # Returns a user-readable name for this target.
    rule full-name ( )
    {
        local location = [ $(self.project).get location ] ;
        return $(location)/$(self.name) ;
    }

    # Generates virtual targets for this abstract target using the specified
    # properties, unless a different value of some feature is required by the
    # target.
    # On success, returns:
    # - a property-set with the usage requirements to be applied to dependants
    # - a list of produced virtual targets, which may be empty.
    # If 'property-set' is empty, performs the default build of this target, in
    # a way specific to the derived class.
    #
    rule generate ( property-set )
    {
        errors.error "method should be defined in derived classes" ;
    }

    rule rename ( new-name )
    {
        self.name = $(new-name) ;
    }
}


if --debug-building in [ modules.peek : ARGV ]
{
    modules.poke : .debug-building : true ;
}


rule indent ( )
{
    return $(.indent:J="") ;
}


rule increase-indent ( )
{
    .indent += "    " ;
}


rule decrease-indent ( )
{
    .indent = $(.indent[2-]) ;
}


#  Project target class (derived from 'abstract-target').
#
#  This class has the following responsibilities:
#  - Maintaining a list of main targets in this project and building them.
#
#  Main targets are constructed in two stages:
#  - When Jamfile is read, a number of calls to 'add-alternative' is made. At
#    that time, alternatives can also be renamed to account for inline targets.
#  - The first time 'main-target' or 'has-main-target' rule is called, all
#    alternatives are enumerated and main targets are created.
#
class project-target : abstract-target
{
    import project ;
    import targets ;
    import path ;
    import print ;
    import property-set ;
    import set ;
    import sequence ;
    import toolset ;
    import "class" : new ;

    rule __init__ ( name : project-module parent-project ?
        : requirements * : default-build * )
    {
        abstract-target.__init__ $(name) : $(__name__) ;

        self.project-module = $(project-module) ;
        self.location = [ project.attribute $(project-module) location ] ;
        self.requirements = $(requirements) ;
        self.default-build = $(default-build) ;

        if $(parent-project)
        {
            inherit $(parent-project) ;
        }
    }

    # This is needed only by the 'make' rule. Need to find a way to make 'make'
    # work without this method.
    #
    rule project-module ( )
    {
        return $(self.project-module) ;
    }

    rule get ( attribute )
    {
        return [ project.attribute $(self.project-module) $(attribute) ] ;
    }

    rule build-dir ( )
    {
        if ! $(self.build-dir)
        {
            self.build-dir = [ get build-dir ] ;
            if ! $(self.build-dir)
            {
                local location = [ $(self.project).get location ] ;
                if $(location)
                {
                    self.build-dir = [ path.join $(location) bin ] ;
                }
                else
                {
                    local id = [ get id  ] ;
                    if $(id)
                    {
                        local rid = [ MATCH ^/(.*) : $(id) ] ;
                        self.build-dir = [ path.join [ project.standalone-build-dir ] $(rid) ] ;
                    }
                    else
                    {
                        errors.error "Could not create build-dir for standalone project $(self.project-module:E=)."
                                   : "Missing project id" ;
                    }
                }
            }
        }
        return $(self.build-dir) ;
    }

    # Generates all possible targets contained in this project.
    #
    rule generate ( property-set * )
    {
        if [ modules.peek : .debug-building ]
        {
            ECHO [ targets.indent ] "building project" [ name ]
                " ('$(__name__)') with" [ $(property-set).raw ] ;
            targets.increase-indent ;
        }

        local usage-requirements = [ property-set.empty ] ;
        local targets ;

        for local t in [ targets-to-build ]
        {
            local g = [ $(t).generate $(property-set) ] ;
            usage-requirements = [ $(usage-requirements).add $(g[1]) ] ;
            targets += $(g[2-]) ;
        }
        targets.decrease-indent ;
        return $(usage-requirements) [ sequence.unique $(targets) ] ;
    }

    # Computes and returns a list of abstract-target instances which must be
    # built when this project is built.
    #
    rule targets-to-build ( )
    {
        local result ;

        if ! $(self.built-main-targets)
        {
            build-main-targets ;
        }

        # Collect all main targets here, except for "explicit" ones.
        for local t in $(self.main-targets)
        {
            if ! [ $(t).name ] in $(self.explicit-targets)
            {
                result += $(t) ;
            }
        }

        # Collect all projects referenced via "projects-to-build" attribute.
        local self-location = [ get location ] ;
        for local pn in [ get projects-to-build ]
        {
            result += [ find $(pn)/ ] ;
        }

        return $(result) ;
    }

    # Add 'target' to the list of targets in this project that should be build
    # only by explicit request
    #
    rule mark-target-as-explicit ( target-name * )
    {
        # Record the name of the target, not instance, since this rule is called
        # before main target instances are created.
        self.explicit-targets += $(target-name) ;
    }

    rule mark-target-as-always ( target-name * )
    {
        # Record the name of the target, not instance, since this rule is called
        # before main target instances are created.
        self.always-targets += $(target-name) ;
    }

    # Add new target alternative
    #
    rule add-alternative ( target-instance )
    {
        if $(self.built-main-targets)
        {
            import errors : error : errors.error ;
            errors.error add-alternative called when main targets are already
                created. : in project [ full-name ] ;
        }
        self.alternatives += $(target-instance) ;
        if ! ( [ $(target-instance).name ] in $(self.alternative-names) )
        {
            self.alternative-names += [ $(target-instance).name ] ;
        }
    }

    # Checks if an alternative was declared for the target.
    # Unlike checking for a main target this does not require
    # building the main targets. And hence can be used in/directly
    # while loading a project.
    #
    rule has-alternative-for-target ( target-name )
    {
        if $(target-name) in $(self.alternative-names)
        {
            return 1 ;
        }
    }

    # Returns a 'main-target' class instance corresponding to 'name'.
    #
    rule main-target ( name )
    {
        if ! $(self.built-main-targets)
        {
            build-main-targets ;
        }
        return $(self.main-target.$(name)) ;
    }

    # Returns whether a main target with the specified name exists.
    #
    rule has-main-target ( name )
    {
        if ! $(self.built-main-targets)
        {
            build-main-targets ;
        }

        if $(self.main-target.$(name))
        {
            return true ;
        }
    }

    # Worker function for the find rule not implementing any caching and simply
    # returning nothing in case the target can not be found.
    #
    rule find-really ( id )
    {
        local result ;
        local current-location = [ get location ] ;

        local split = [ MATCH ^(.*)//(.*)$ : $(id) ] ;
        local project-part = $(split[1]) ;
        local target-part = $(split[2]) ;

        local extra-error-message ;
        if $(project-part)
        {
            # There is an explicitly specified project part in id. Looks up the
            # project and passes the request to it.
            local pm = [ project.find $(project-part) : $(current-location) ] ;
            if $(pm)
            {
                project-target = [ project.target $(pm) ] ;
                result = [ $(project-target).find $(target-part) : no-error ] ;
            }
            else
            {
                extra-error-message = could not resolve project reference
                    '$(project-part)' ;
                if ! [ path.is-rooted $(project-part) ]
                {
                    local rooted = [ path.root $(project-part) / ] ;
                    if $(rooted) && [ project.is-registered-id $(rooted) ]
                    {
                        extra-error-message += - possibly missing a leading
                            slash ('/') character. ;
                    }
                }
            }
        }
        else
        {
            # Interpret target-name as name of main target. Need to do this
            # before checking for file. Consider the following scenario with a
            # toolset not modifying its executable's names, e.g. gcc on
            # Unix-like platforms:
            #
            #  exe test : test.cpp ;
            #  install s : test : <location>. ;
            #
            # After the first build we would have a target named 'test' in the
            # Jamfile and a file named 'test' on the disk. We need the target to
            # override the file.
            result = [ main-target $(id) ] ;

            # Interpret id as an existing file reference.
            if ! $(result)
            {
                result = [ new file-reference [ path.make $(id) ] :
                    $(self.project) ] ;
                if ! [ $(result).exists ]
                {
                    result = ;
                }
            }

            # Interpret id as project-id.
            if ! $(result)
            {
                local project-module = [ project.find $(id) :
                    $(current-location) ] ;
                if $(project-module)
                {
                    result = [ project.target $(project-module) ] ;
                }
            }
        }

        return $(result:E="") $(extra-error-message) ;
    }

    # Find and return the target with the specified id, treated relative to
    # self. Id may specify either a target or a file name with the target taking
    # priority. May report an error or return nothing if the target is not found
    # depending on the 'no-error' parameter.
    #
    rule find ( id : no-error ? )
    {
        local v = $(.id.$(id)) ;
        local extra-error-message ;
        if ! $(v)
        {
            local r = [ find-really $(id) ] ;
            v = $(r[1]) ;
            extra-error-message = $(r[2-]) ;
            if ! $(v)
            {
                v = none ;
            }
            .id.$(id) = $(v) ;
        }

        if $(v) != none
        {
            return $(v) ;
        }
        else if ! $(no-error)
        {
            local current-location = [ get location ] ;
            import errors : user-error : errors.user-error ;
            errors.user-error Unable to find file or target named
                : "   " '$(id)'
                : referred to from project at
                : "   " '$(current-location)'
                : $(extra-error-message) ;
        }
    }

    rule build-main-targets ( )
    {
        self.built-main-targets = true ;
        for local a in $(self.alternatives)
        {
            local name = [ $(a).name ] ;
            local target = $(self.main-target.$(name)) ;
            if ! $(target)
            {
                local t = [ new main-target $(name) : $(self.project) ] ;
                self.main-target.$(name) = $(t) ;
                self.main-targets += $(t) ;
                target = $(self.main-target.$(name)) ;
            }

            if $(name) in $(self.always-targets)
            {
                $(a).always ;
            }

            $(target).add-alternative $(a) ;
        }
    }

    # Accessor, add a constant.
    #
    rule add-constant (
        name       # Variable name of the constant.
        : value +  # Value of the constant.
        : type ?   # Optional type of value.
        )
    {
        switch $(type)
        {
            case path :
              local r ;
              for local v in $(value)
              {
                local l = $(self.location) ;
                if ! $(l)
                {
                    # Project corresponding to config files do not have
                    # 'location' attribute, but do have source location. It
                    # might be more reasonable to make every project have a
                    # location and use some other approach to prevent buildable
                    # targets in config files, but that has been left for later.
                    l = [ get source-location ] ;
                }
                v = [ path.root [ path.make $(v) ] $(l) ] ;
                # Now make the value absolute path.
                v = [ path.root $(v) [ path.pwd ] ] ;
                # Constants should be in platform-native form.
                v = [ path.native $(v) ] ;
                r += $(v) ;
              }
              value = $(r) ;
        }
        if ! $(name) in $(self.constants)
        {
            self.constants += $(name) ;
        }
        self.constant.$(name) = $(value) ;
        # Inject the constant in the scope of the Jamroot module.
        modules.poke $(self.project-module) : $(name) : $(value) ;
    }

    rule inherit ( parent )
    {
        for local c in [ modules.peek $(parent) : self.constants ]
        {
            # No need to pass the type. Path constants were converted to
            # absolute paths already by parent.
            add-constant $(c) : [ modules.peek $(parent) : self.constant.$(c) ]
                ;
        }

        # Import rules from parent.
        local this-module = [ project-module ] ;
        local parent-module = [ $(parent).project-module ] ;
        # Do not import rules coming from 'project-rules' as they must be
        # imported localized.
        local user-rules = [ set.difference
            [ RULENAMES $(parent-module) ] :
            [ RULENAMES project-rules ] ] ;
        IMPORT $(parent-module) : $(user-rules) : $(this-module) : $(user-rules)
            ;
        EXPORT $(this-module) : $(user-rules) ;

        toolset.inherit-flags $(this-module) : $(parent-module) ;
    }
}


# Helper rules to detect cycles in main target references.
#
local rule start-building ( main-target-instance )
{
    if $(main-target-instance) in $(.targets-being-built)
    {
        local names ;
        for local t in $(.targets-being-built) $(main-target-instance)
        {
            names += [ $(t).full-name ] ;
        }

        import errors ;
        errors.error "Recursion in main target references"
          : "the following target are being built currently:"
          : $(names) ;
    }
    .targets-being-built += $(main-target-instance) ;
}


local rule end-building ( main-target-instance )
{
    .targets-being-built = $(.targets-being-built[1--2]) ;
}


# A named top-level target in Jamfile.
#
class main-target : abstract-target
{
    import assert ;
    import feature ;
    import property-set ;
    import sequence ;
    import set ;
    import targets : start-building end-building ;
    import utility ;

    rule __init__ ( name : project )
    {
        abstract-target.__init__ $(name) : $(project) ;
    }

    # Add a new alternative for this target
    rule add-alternative ( target )
    {
        local d = [ $(target).default-build ] ;
        if $(self.alternatives) && ( $(self.default-build) != $(d) )
        {
            import errors : error : errors.error ;
            errors.error "default build must be identical in all alternatives"
                : "main target is" [ full-name ]
                : "with" [ $(d).raw ]
                : "differing from previous default build"
                    [ $(self.default-build).raw ] ;
        }
        else
        {
            self.default-build = $(d) ;
        }
        self.alternatives += $(target) ;
    }

    # Returns the best viable alternative for this property-set. See the
    # documentation for selection rules.
    #
    rule select-alternatives ( property-set debug ? )
    {
        # When selecting alternatives we have to consider defaults, for example:
        #    lib l : l.cpp : <variant>debug ;
        #    lib l : l_opt.cpp : <variant>release ;
        # will not work unless we add default value <variant>debug.
        property-set = [ $(property-set).add-defaults ] ;

        # The algorithm: we keep the current best viable alternative. When we
        # encounter a new best viable alternative, we compare it with the
        # current one.

        local best ;
        local best-properties ;

        if $(self.alternatives[2-])
        {
            local bad ;
            local worklist = $(self.alternatives) ;
            while $(worklist) && ! $(bad)
            {
                local v = $(worklist[1]) ;
                local properties = [ $(v).match $(property-set) $(debug) ] ;

                if $(properties) != no-match
                {
                    if ! $(best)
                    {
                        best = $(v) ;
                        best-properties = $(properties) ;
                    }
                    else
                    {
                        if $(properties) = $(best-properties)
                        {
                            bad = true ;
                        }
                        else if $(properties) in $(best-properties)
                        {
                            # Do nothing, this alternative is worse
                        }
                        else if $(best-properties) in $(properties)
                        {
                            best = $(v) ;
                            best-properties = $(properties) ;
                        }
                        else
                        {
                            bad = true ;
                        }
                    }
                }
                worklist = $(worklist[2-]) ;
            }
            if ! $(bad)
            {
                return $(best) ;
            }
        }
        else
        {
            return $(self.alternatives) ;
        }
    }

    # Features are relevant here if they could affect alternative
    # selection.  That is, base, non-conditional properties that
    # are not identical in all target alternatives.
    rule relevant-features ( )
    {
        if $(self.alternatives[2-])
        {
            if $(self.relevant-features)
            {
                return $(self.relevant-features) ;
            }
            local all-properties ;
            for t in $(self.alternatives)
            {
                local ps = [ $(t).requirements ] ;
                ps = [ property-set.create [ $(ps).non-conditional ] ] ;
                all-properties += [ $(ps).base ] ;
            }
            all-properties = [ sequence.unique $(all-properties) ] ;
            local result ;
            for t in $(self.alternatives)
            {
                local ps = [ $(t).requirements ] ;
                ps = [ property-set.create [ $(ps).non-conditional ] ] ;
                local properties = [ set.difference $(all-properties) : [ $(ps).base ] ] ;
                result += $(properties:G) ;
            }
            result = [ sequence.transform utility.ungrist : [ sequence.unique $(result) ] ] ;
            self.relevant-features = [ property-set.create <relevant>$(result) ] ;
            return $(self.relevant-features) ;
        }
        else
        {
            return [ property-set.empty ] ;
        }
    }

    rule apply-default-build ( property-set )
    {
        return [ targets.apply-default-build $(property-set) :
            $(self.default-build) ] ;
    }

    # Select an alternative for this main target, by finding all alternatives
    # whose requirements are satisfied by 'properties' and picking the one with
    # the longest requirements set. Returns the result of calling 'generate' on
    # that alternative.
    #
    rule generate ( property-set )
    {
        start-building $(__name__) ;

        local all-property-sets = [ apply-default-build $(property-set) ] ;
        local relevant = [ relevant-features ] ;
        local usage-requirements = [ property-set.empty ] ;
        local result ;
        for local p in $(all-property-sets)
        {
            local r = [ generate-really [ $(p).add $(relevant) ] ] ;
            if $(r)
            {
                usage-requirements = [ $(usage-requirements).add $(r[1]) ] ;
                result += $(r[2-]) ;
            }
        }
        end-building $(__name__) ;
        return $(usage-requirements) [ sequence.unique $(result) ] ;
    }

    # Generates the main target with the given property set and returns a list
    # which first element is property-set object containing usage-requirements
    # of generated target and with generated virtual target in other elements.
    # It is possible that no targets are generated.
    #
    local rule generate-really ( property-set )
    {
        local best-alternatives = [ select-alternatives $(property-set) ] ;
        if ! $(best-alternatives)
        {
            ECHO "error: No best alternative for" [ full-name ] ;
            select-alternatives $(property-set) debug ;
            return [ property-set.empty ] ;
        }
        else
        {
            # Now return virtual targets for the only alternative.
            return [ $(best-alternatives).generate $(property-set) ] ;
        }
    }

    rule rename ( new-name )
    {
        abstract-target.rename $(new-name) ;
        for local a in $(self.alternatives)
        {
            $(a).rename $(new-name) ;
        }
    }
}


# Abstract target referring to a source file. This is an artificial entity
# allowing sources to a target to be represented using a list of abstract target
# instances.
#
class file-reference : abstract-target
{
    import virtual-target ;
    import property-set ;
    import path ;

    rule __init__ ( file : project )
    {
        abstract-target.__init__ $(file) : $(project) ;
    }

    rule generate ( properties )
    {
        return [ property-set.empty ] [ virtual-target.from-file $(self.name) :
            [ location ] : $(self.project) ] ;
    }

    # Returns true if the referred file really exists.
    rule exists ( )
    {
        location ;
        return $(self.file-path) ;
    }

    # Returns the location of target. Needed by 'testing.jam'.
    rule location ( )
    {
        if ! $(self.file-location)
        {
            local source-location = [ $(self.project).get source-location ] ;
            for local src-dir in $(source-location)
            {
                if ! $(self.file-location)
                {
                    local location = [ path.root $(self.name) $(src-dir) ] ;
                    if [ CHECK_IF_FILE [ path.native $(location) ] ]
                    {
                         self.file-location = $(src-dir) ;
                         self.file-path = $(location) ;
                    }
                }
            }
        }
        return $(self.file-location) ;
    }
}


# Given a target-reference, made in context of 'project', returns the
# abstract-target instance that is referred to, as well as properties explicitly
# specified for this reference.
#
rule resolve-reference ( target-reference : project )
{
    # Separate target name from properties override.
    local split = [ MATCH "^([^<]*)(/(<.*))?$" : $(target-reference) ] ;
    local id = $(split[1]) ;
    if ! $(split) || ! $(id)
    {
        import errors ;
        errors.error "Malformed target reference $(target-reference)" ;
    }
    local sproperties = ;
    if $(split[3])
    {
        sproperties = [ property.make [ feature.split $(split[3]) ] ] ;
        sproperties = [ feature.expand $(sproperties) ] ;
    }

    # Find the target.
    local target = [ $(project).find $(id) ] ;

    return $(target) [ property-set.create $(sproperties) ] ;
}


# Attempts to generate the target given by target reference, which can refer
# both to a main target or to a file. Returns a list consisting of
# - usage requirements
# - generated virtual targets, if any
#
rule generate-from-reference (
    target-reference  # Target reference.
    : project         # Project where the reference is made.
    : property-set    # Properties of the main target that makes the reference.
)
{
    local r = [ resolve-reference $(target-reference) : $(project) ] ;
    local target = $(r[1]) ;
    local sproperties = $(r[2]) ;

    # Take properties which should be propagated and refine them with
    # source-specific requirements.
    local propagated = [ $(property-set).propagated ] ;
    local rproperties = [ $(propagated).refine $(sproperties) ] ;
    if $(rproperties[1]) = "@error"
    {
        import errors ;
        errors.error
            "When building" [ full-name ] " with properties " $(properties) :
            "Invalid properties specified for " $(source) ":"
            $(rproperties[2-]) ;
    }
    return [ $(target).generate $(rproperties) ] ;
}


rule apply-default-build ( property-set : default-build )
{
    # 1. First, see what properties from default-build are already present in
    # property-set.

    local expanded = [ $(property-set).expand ] ;
    local raw = [ $(property-set).raw ] ;
    local specified-features = [ $(expanded).raw ] ;
    specified-features = $(specified-features:G) ;

    local defaults-to-apply ;
    for local d in [ $(default-build).raw ]
    {
        if ! $(d:G) in $(specified-features)
        {
            defaults-to-apply += $(d) ;
        }
    }

    # 2. If there are any defaults to be applied, form a new build request. Pass
    # it through to 'expand-no-defaults' since default-build might contain
    # "release debug" resulting in two property-sets.
    local result ;
    if $(defaults-to-apply)
    {
        # We have to compress subproperties here to prevent property lists like:
        #    <toolset>msvc <toolset-msvc:version>7.1 <threading>multi
        #
        # from being expanded into:
        #    <toolset-msvc:version>7.1/<threading>multi
        #    <toolset>msvc/<toolset-msvc:version>7.1/<threading>multi
        #
        # due to a cross-product property combination. That may be an indication
        # that build-request.expand-no-defaults is the wrong rule to use here.
        properties = [ build-request.expand-no-defaults
            [ feature.compress-subproperties $(raw) ] $(defaults-to-apply) ] ;

        if $(properties)
        {
            for local p in $(properties)
            {
                result += [ property-set.create
                    [ feature.split $(p) ] ] ;
            }
        }
        else
        {
            result = [ property-set.empty ] ;
        }
    }
    else
    {
        result = $(property-set) ;
    }
    return $(result) ;
}


# Given a build request and requirements, return properties common to dependency
# build request and target requirements.
#
# TODO: Document exactly what 'common properties' are, whether they should
# include default property values, whether they should contain any conditional
# properties or should those be already processed, etc. See whether there are
# any differences between use cases with empty and non-empty build-request as
# well as with requirements containing and those not containing any non-free
# features.
#
rule common-properties ( build-request requirements )
{
    # For optimization, we add free requirements directly, without using a
    # complex algorithm. This gives the complex algorithm a better chance of
    # caching results.
    local free = [ $(requirements).free ] ;
    local non-free = [ property-set.create [ $(requirements).base ]
        [ $(requirements).incidental ] ] ;

    local key = .rp.$(build-request)-$(non-free) ;
    if ! $($(key))
    {
        $(key) = [ common-properties2 $(build-request) $(non-free) ] ;
    }
    return [ $($(key)).add-raw $(free) ] ;
}


# Given a 'context' -- a set of already present properties, and 'requirements',
# decide which extra properties should be applied to 'context'. For conditional
# requirements, this means evaluating the condition. For indirect conditional
# requirements, this means calling a rule. Ordinary requirements are always
# applied.
#
# Handles the situation where evaluating one conditional requirement affects
# conditions of another conditional requirements, such as:
#     <toolset>gcc:<variant>release <variant>release:<define>RELEASE
#
# If 'what' is 'refined' returns context refined with new requirements. If
# 'what' is 'added' returns just the requirements to be applied.
#
rule evaluate-requirements ( requirements : context )
{
    # Apply non-conditional requirements. It is possible that further
    # conditional requirement change a value set by non-conditional
    # requirements. For example:
    #
    #    exe a : a.cpp : <threading>single <toolset>foo:<threading>multi ;
    #
    # I am not sure if this should be an error, or not, especially given that
    #
    #    <threading>single
    #
    # might come from project's requirements.

    local unconditional = [ feature.expand [ $(requirements).non-conditional ] ]
        ;

    local raw = [ $(context).raw ] ;
    raw = [ property.refine $(raw) : $(unconditional) ] ;

    # We have collected properties that surely must be present in common
    # properties. We now try to figure out what other properties should be added
    # in order to satisfy rules (4)-(6) from the docs.

    local defaults = [ toolset.defaults ] ;
    defaults = [ $(defaults).raw ] ;

    local conditionals = [ $(requirements).conditional ] ;
    # The 'count' variable has one element for each conditional feature and for
    # each occurrence of '<conditional>' feature. It is used as a loop
    # counter: for each iteration of the loop before we remove one element and
    # the property set should stabilize before we are done. It is assumed that
    # #conditionals iterations should be enough for properties to propagate
    # along conditions in any direction.
    local count = $(conditionals) $(defaults)
        and-once-more ;

    local added-requirements ;
    local added-defaults ;

    local current = $(raw) ;

    local ok ;
    while $(count)
    {
        # We need to expand composites here so that the requirements can
        # safely override composite features.
        current = [ feature.expand-composites $(current) ] ;
        current = [ feature.add-defaults $(current) ] ;
        # Evaluate conditionals in context of current properties.
        local e = [ property.evaluate-conditionals-in-context $(conditionals) :
            $(current) ] ;
        local d = [ property.evaluate-conditionals-in-context $(defaults) :
            $(current) ] ;

        if $(e) = $(added-requirements) && $(d) = $(added-defaults)
        {
            # If we got the same result, we have found the final properties.
            count = ;
            ok = true ;
        }
        else
        {
            # Oops, conditional evaluation results have changed. Also 'current'
            # contains leftovers from a previous evaluation. Recompute 'current'
            # using initial properties and conditional requirements.
            added-requirements = $(e) ;
            current = [ property.refine $(raw) : [ feature.expand $(e) ] ] ;
            added-defaults = $(d) ;
            current = [ property.refine $(d) : $(current) ] ;
        }
        count = $(count[2-]) ;
    }
    if ! $(ok)
    {
        import errors ;
        errors.error Can not evaluate conditional properties $(conditionals) ;
    }

    return [ property-set.create $(current) ] ;
}


rule common-properties2 ( build-request requirements )
{
    return [ evaluate-requirements $(requirements) : $(build-request) ] ;
}


rule push-target ( target )
{
    .targets = $(target) $(.targets) ;
}

rule pop-target ( )
{
    .targets = $(.targets[2-]) ;
}

# Return the metatarget that is currently being generated.
rule current ( )
{
    return $(.targets[1]) ;
}


# Implements the most standard way of constructing main target alternative from
# sources. Allows sources to be either file or other main target and handles
# generation of those dependency targets.
#
class basic-target : abstract-target
{
    import build-request ;
    import build-system ;
    import "class" : new ;
    import feature ;
    import property ;
    import property-set ;
    import sequence ;
    import set ;
    import targets ;
    import virtual-target ;

    rule __init__ ( name : project : sources * : requirements * :
        default-build * : usage-requirements * )
    {
        abstract-target.__init__ $(name) : $(project) ;

        self.sources = $(sources) ;
        if ! $(requirements)
        {
            requirements = [ property-set.empty ] ;
        }
        self.requirements = $(requirements) ;
        if ! $(default-build)
        {
            default-build = [ property-set.empty ] ;
        }
        self.default-build = $(default-build) ;
        if ! $(usage-requirements)
        {
            usage-requirements = [ property-set.empty ] ;
        }
        self.usage-requirements = $(usage-requirements) ;

        if $(sources:G)
        {
            import errors : user-error : errors.user-error ;
            errors.user-error properties found "in" the 'sources' parameter
                "for" [ full-name ] ;
        }
    }

    rule always ( )
    {
        self.always = 1 ;
    }

    # Returns the list of abstract-targets which are used as sources. The extra
    # properties specified for sources are not represented. The only user for
    # this rule at the moment is the "--dump-tests" feature of the test system.
    #
    rule sources ( )
    {
        if ! $(self.source-targets)
        {
            for local s in $(self.sources)
            {
                self.source-targets += [ targets.resolve-reference $(s) :
                    $(self.project) ] ;
            }
        }
        return $(self.source-targets) ;
    }

    rule requirements ( )
    {
        return $(self.requirements) ;
    }

    rule default-build ( )
    {
        return $(self.default-build) ;
    }

    # Returns the alternative condition for this alternative, if the condition
    # is satisfied by 'property-set'.
    #
    rule match ( property-set debug ? )
    {
        # The condition is composed of all base non-conditional properties. We
        # only expand subfeatures in the condition.  We do not expand
        # composites.  We want to be able to put
        #    <toolset>msvc-6.0
        # in requirements. On the other hand, if we have <variant>release as a
        # condition it does not make sense to require <optimization>full to be
        # in the build request just to select this variant.
        local bcondition = [ $(self.requirements).base ] ;
        local ccondition = [ $(self.requirements).conditional ] ;
        local condition = [ set.difference $(bcondition) : $(ccondition) ] ;
        condition = [ feature.expand-subfeatures $(condition) : unchecked ] ;
        if $(debug)
        {
            ECHO "    next alternative: required properties:"
                $(condition:E=(empty)) ;
        }

        if $(condition) in [ $(property-set).raw ]
        {
            if $(debug)
            {
                ECHO "        matched" ;
            }
            return $(condition) ;
        }
        else
        {
            if $(debug)
            {
                ECHO "        not matched" ;
            }
            return no-match ;
        }
    }

    # Takes a target reference, which might be either target id or a dependency
    # property, and generates that target using 'property-set' as a build
    # request.
    #
    # The results are added to the variable called 'result-var'. Usage
    # requirements are added to the variable called 'usage-requirements-var'.
    #
    rule generate-dependencies ( dependencies * : property-set : result-var
        usage-requirements-var )
    {
        for local dependency in $(dependencies)
        {
            local grist = $(dependency:G) ;
            local id = $(dependency:G=) ;
            local result = [ targets.generate-from-reference $(id) :
                $(self.project) : $(property-set) ] ;

            $(result-var) += $(result[2-]:G=$(grist)) ;
            $(usage-requirements-var) += [ $(result[1]).raw ] ;
        }
    }

    # Determines final build properties, generates sources, and calls
    # 'construct'. This method should not be overridden.
    #
    rule generate ( property-set )
    {
        if [ modules.peek : .debug-building ]
        {
            ECHO ;
            local fn = [ full-name ] ;
            ECHO [ targets.indent ] "Building target '$(fn)'" ;
            targets.increase-indent ;
            ECHO [ targets.indent ] Build "request:" $(property-set)
                [ $(property-set).raw ] ;
            local cf = [ build-system.command-line-free-features ] ;
            ECHO [ targets.indent ] Command line free "features:" [ $(cf).raw ] ;
            ECHO [ targets.indent ] Target "requirements:"
                [ $(self.requirements).raw ] ;
        }
        targets.push-target $(__name__) ;

        # Apply free features from the command line. If user said
        #   define=FOO
        # he most likely wants this define to be set for all compiles.
        # Make it before check for already built.
        property-set = [ $(property-set).add
            [ build-system.command-line-free-features ] ] ;

        if ! $(self.generated.$(property-set))
        {
            local rproperties = [ targets.common-properties $(property-set)
                $(self.requirements) ] ;

            if [ modules.peek : .debug-building ]
            {
                ECHO ;
                ECHO [ targets.indent ] "Common properties: "
                    [ $(rproperties).raw ] ;
            }

            local skip ;
            local skip-downstream ;
            if $(rproperties[1]) = "@error"
            {
                ECHO [ targets.indent ] "Skipping build of:" [ full-name ]
                    "cannot compute common properties" ;
                skip = true ;
                skip-downstream = true ;
            }
            else if [ $(rproperties).get <build> ] = no
            {
                # If we just see <build>no, we cannot produce any reasonable
                # diagnostics. The code that adds this property is expected
                # to explain why a target is not built, for example using
                # the configure.log-component-configuration function.
                skip = true ;
                skip-downstream = true ;
            }
            else
            {
                local source-targets ;
                local properties = [ $(rproperties).non-dependency ] ;
                local usage-requirements ;

                generate-dependencies [ $(rproperties).dependency ] :
                    $(rproperties) : properties usage-requirements ;

                generate-dependencies $(self.sources) : $(rproperties) :
                    source-targets usage-requirements ;

                if [ modules.peek : .debug-building ]
                {
                    ECHO ;
                    ECHO [ targets.indent ] "Usage requirements for"
                        $(self.name)": " $(usage-requirements) ;
                }

                #  Skipping this target if a dependency is skipped.
                #  Subclasses can override this behavior. E.g.
                #  alias-target-class overrides this to not be skipped if a
                #  dependency is skipped.
                if <build>no in $(usage-requirements)
                {
                    skip = [ skip-from-usage-requirements ] ;
                    skip-downstream = true ;
                }

                if ! $(skip)
                {
                    rproperties = [ property-set.create $(properties)
                        $(usage-requirements) ] ;
                    usage-requirements = [ property-set.create $(usage-requirements)
                        ] ;

                    if [ modules.peek : .debug-building ]
                    {
                        ECHO [ targets.indent ] "Build properties: "
                            [ $(rproperties).raw ] ;
                    }

                    local extra = [ $(rproperties).get <source> ] ;
                    source-targets += $(extra:G=) ;
                    # We might get duplicate sources, for example if we link to two
                    # libraries having the same <library> usage requirement. Use
                    # stable sort, since for some targets the order is important,
                    # e.g. RUN_PY targets need a python source to come first.
                    source-targets = [ sequence.unique $(source-targets) : stable ]
                        ;

                    local result = [ construct $(self.name) : $(source-targets) :
                        $(rproperties) ] ;

                    if $(result)
                    {
                        local gur = $(result[1]) ;
                        result = $(result[2-]) ;

                        # Relevant is automatically applied to usage requirements
                        # and only applies for propagated features
                        local relevant = [ propagated-relevant
                            [ $(gur).get <relevant> ]
                            [ $(rproperties).get <relevant> ] ] ;
                        gur = [ property-set.create
                            [ property.change [ $(gur).raw ] : <relevant> ]
                            <relevant>$(relevant) ] ;

                        local s = [ create-subvariant $(result)
                            : [ virtual-target.recent-targets ]
                            : $(property-set) : $(source-targets)
                            : $(rproperties) : $(usage-requirements) ] ;
                        virtual-target.clear-recent-targets ;

                        if $(self.always)
                        {
                            for local t in [ $(s).created-targets ]
                            {
                                $(t).always ;
                            }
                        }

                        local ur = [ compute-usage-requirements $(s) ] ;
                        if $(skip-downstream)
                        {
                            ur = [ $(ur).add [ property-set.create <build>no ]
                                ] ;
                        }
                        ur = [ $(ur).add $(gur) ] ;
                        $(s).set-usage-requirements $(ur) ;
                        if [ modules.peek : .debug-building ]
                        {
                            ECHO [ targets.indent ] "Usage requirements from"
                                $(self.name)": " [ $(ur).raw ] ;
                        }

                        self.generated.$(property-set) = $(ur) $(result) ;
                    }
                }
            }

            if $(skip)
            {
                # We are here either because there has been an error computing
                # properties or there is <build>no in properties or usage
                # requirements. In the latter case we do not want any
                # diagnostic. In the former case, we need diagnostics. FIXME

                # If this target fails to build, add <build>no to properties to
                # cause any parent target to fail to build.
                self.generated.$(property-set) = [ property-set.create <build>no
                    ] ;
            }
        }
        else
        {
            if [ modules.peek : .debug-building ]
            {
                ECHO [ targets.indent ] "Already built" ;
                local ur = $(self.generated.$(property-set)) ;
                ur = $(ur[0]) ;
                targets.increase-indent ;
                ECHO [ targets.indent ] "Usage requirements from"
                    $(self.name)": " [ $(ur).raw ] ;
                targets.decrease-indent ;
            }
        }

        targets.pop-target ;
        targets.decrease-indent ;
        return $(self.generated.$(property-set)) ;
    }

    # Given the set of generated targets, and refined build properties,
    # determines and sets appropriate usage requirements on those targets.
    #
    rule compute-usage-requirements ( subvariant )
    {
        local rproperties = [ $(subvariant).build-properties ] ;
        xusage-requirements =
            [ $(self.usage-requirements).evaluate-conditionals
              $(rproperties) ] ;

        # Filter out non-propagated <relevant> properties
        local relevant ;
        for local r in [ $(xusage-requirements).get <relevant> ]
        {
            local check = [ MATCH "(.*):<relevant>(.*)" : $(r) ] ;
            if $(check) { check = $(check[2]) ; }
            else { check = $(r) ; }
            if propagated in [ feature.attributes <$(check)> ]
            {
                relevant += $(r) ;
            }
        }
        local raw = [ $(xusage-requirements).raw ] ;
        local free = [ property.take free : $(raw) ] ;
        if $(free) != $(raw)
        {
            if ! $(self.warned-usage-requirements)
            {
                self.warned-usage-requirements = true ;
                ECHO "warning:" non-free usage requirements
                    [ set.difference $(raw) : $(free) ] ignored ;
                ECHO "warning:" in main-target [ name ] at [ location ] ;
            }
        }
        xusage-requirements = [ property-set.create
            [ property.change $(free) : <relevant> ]
            <relevant>$(relevant) ] ;

        # We generate all dependency properties and add them, as well as their
        # usage requirements, to the result.
        local extra ;
        generate-dependencies [ $(xusage-requirements).dependency ] :
            $(rproperties) : extra extra ;

        local result = [ property-set.create
            [ $(xusage-requirements).non-dependency ] $(extra) ] ;

        # Propagate usage requirements we got from sources, except for the
        # <pch-header> and <pch-file> features.
        #
        # That feature specifies which pch file to use, and should apply only to
        # direct dependents. Consider:
        #
        #   pch pch1 : ...
        #   lib lib1 : ..... pch1 ;
        #   pch pch2 :
        #   lib lib2 : pch2 lib1 ;
        #
        # Here, lib2 should not get <pch-header> property from pch1.
        #
        # Essentially, when those two features are in usage requirements, they
        # are propagated only to direct dependents. We might need a more general
        # mechanism, but for now, only those two features are special.
        #
        # TODO - Actually there are more possible candidates like for instance
        # when listing static library X as a source for another static library.
        # Then static library X will be added as a <source> property to the
        # second library's usage requirements but those requirements should last
        # only up to the first executable or shared library that actually links
        # to it.
        local raw = [ $(subvariant).sources-usage-requirements ] ;
        raw = [ $(raw).raw ] ;
        raw = [ property.change $(raw) : <pch-header> ] ;
        raw = [ property.change $(raw) : <pch-file> ] ;
        return [ $(result).add [ property-set.create $(raw) ] ] ;
    }

    local rule propagated-relevant ( values * )
    {
        local result ;
        for local v in [ feature.expand-relevant $(values) ]
        {
            if propagated in [ feature.attributes <$(v)> ]
            {
                result += $(v) ;
            }
        }
        return $(result) ;
    }

    # Creates new subvariant instances for 'targets'.
    # 'root-targets'  - virtual targets to be returned to dependants
    # 'all-targets'   - virtual targets created while building this main target
    # 'build-request' - property-set instance with requested build properties
    #
    local rule create-subvariant ( root-targets * : all-targets * :
        build-request : sources * : rproperties : usage-requirements )
    {
        for local e in $(root-targets)
        {
            $(e).root true ;
        }

        # Process all virtual targets that will be created if this main target
        # is created.
        local s = [ new subvariant $(__name__) : $(build-request) : $(sources) :
            $(rproperties) : $(usage-requirements) : $(all-targets) ] ;
        for local v in $(all-targets)
        {
            if ! [ $(v).creating-subvariant ]
            {
                $(v).creating-subvariant $(s) ;
            }
        }
        return $(s) ;
    }

    # Constructs virtual targets for this abstract target and the dependency
    # graph. Returns a usage-requirements property-set and a list of virtual
    # targets. Should be overridden in derived classes.
    #
    rule construct ( name : source-targets * : properties * )
    {
        import errors : error : errors.error ;
        errors.error "method should be defined in derived classes" ;
    }

    # Determines if build of this target should be skipped when there is
    # <build>no in usage requirements. This should usually be true, unless
    # the target is some kind of grouping, e.g. alias targets.
    rule skip-from-usage-requirements ( )
    {
        return true ;
    }
}


class typed-target : basic-target
{
    import generators ;

    rule __init__ ( name : project : type : sources * : requirements * :
         default-build * : usage-requirements * )
    {
        basic-target.__init__ $(name) : $(project) : $(sources) :
            $(requirements) : $(default-build) : $(usage-requirements) ;

        self.type = $(type) ;
    }

    rule type ( )
    {
        return $(self.type) ;
    }

    rule construct ( name : source-targets * : property-set )
    {
        local r = [ generators.construct $(self.project) $(name:S=)
            : $(self.type)
            : [ property-set.create [ $(property-set).raw ]
                <main-target-type>$(self.type) ]
            : $(source-targets) : true ] ;
        if ! $(r)
        {
            local viable-generators = [ generators.find-viable-generators
                $(self.type) : $(property-set) ] ;
            ECHO "WARNING: Unable to construct" [ full-name ]
                "of type" $(self.type)
                "with these properties:" [ $(property-set).raw ] ;
            ECHO "WARNING: Considered these as possible generators:" ;
            for local gen in $(viable-generators)
            {
                ECHO "WARNING:" [ $(gen).id ]
                    "with source types {" [ $(gen).source-types ] "}"
                    "and requirements {" [ $(gen).requirements ] "}" ;
            }

            # Are there any top-level generators for this type/property set.
            if ! [ generators.find-viable-generators $(self.type) :
                $(property-set) ]
            {
                ECHO "error: no generators were found for type '$(self.type)'" ;
                ECHO "error: and the requested properties" ;
                ECHO "error: make sure you've configured the needed tools" ;
                ECHO "See https://www.bfgroup.xyz/b2/manual/release/index.html#bbv2.overview.configuration" ;
                EXIT "To debug this problem, try the --debug-generators option."
                    ;
            }
        }
        return $(r) ;
    }
}


# Return the list of sources to use, if main target rule is invoked with
# 'sources'. If there are any objects in 'sources', they are treated as main
# target instances, and the name of such targets are adjusted to be
# '<name_of_this_target>__<name_of_source_target>'. Such renaming is disabled if
# a non-empty value is passed as the 'no-renaming' parameter.
#
rule main-target-sources ( sources * : main-target-name : no-renaming ? )
{
    local result ;
    for local t in $(sources)
    {
        if [ class.is-instance $(t) ]
        {
            local name = [ $(t).name ] ;
            if ! $(no-renaming)
            {
                name = $(main-target-name)__$(name) ;
                $(t).rename $(name) ;
            }
            # Inline targets are not built by default.
            local p = [ $(t).project ] ;
            $(p).mark-target-as-explicit $(name) ;
            result += $(name) ;
        }
        else
        {
            result += $(t) ;
        }
    }
    return $(result) ;
}


# Returns the requirements to use when declaring a main target, obtained by
# translating all specified property paths and refining project requirements
# with the ones specified for the target.
#
rule main-target-requirements (
    specification *  # Properties explicitly specified for the main target.
    : project        # Project where the main target is to be declared.
)
{
    local requirements = [ property-set.refine-from-user-input
        [ $(project).get requirements ] : $(specification) :
        [ $(project).project-module ] : [ $(project).get location ] ] ;
    if $(requirements[1]) = "@error"
    {
        import errors ;
        errors.error "Conflicting requirements for target:" $(requirements) ;
    }
    local result = [ $(requirements).add [ toolset.requirements ] ] ;
    return [ $(result).add-raw [ property.evaluate-conditional-relevance [ $(result).raw ] ] ] ;
}


# Returns the usage requirements to use when declaring a main target, which are
# obtained by translating all specified property paths and adding project's
# usage requirements.
#
rule main-target-usage-requirements (
    specification *  # Use-properties explicitly specified for a main target.
    : project        # Project where the main target is to be declared.
)
{
    local project-usage-requirements = [ $(project).get usage-requirements ] ;

    # We do not use 'refine-from-user-input' because:
    # - I am not sure if removing parent's usage requirements makes sense
    # - refining usage requirements is not needed, since usage requirements are
    #   always free.
    local usage-requirements = [ property-set.create-from-user-input
        $(specification)
        : [ $(project).project-module ] [ $(project).get location ] ] ;

    local result = [ $(project-usage-requirements).add $(usage-requirements) ] ;
    local relevant =
      [ property.evaluate-conditional-relevance [ $(result).raw ] ] ;
    return [ $(result).add-raw $(relevant) ] ;
}


# Return the default build value to use when declaring a main target, which is
# obtained by using the specified value if not empty and parent's default build
# attribute otherwise.
#
rule main-target-default-build (
    specification *  # Default build explicitly specified for a main target.
    : project        # Project where the main target is to be declared.
)
{
    local result ;
    if $(specification)
    {
        result = $(specification) ;
    }
    else
    {
        result = [ $(project).get default-build ] ;
    }
    return [ property-set.create-with-validation $(result) ] ;
}


# Registers the specified target as a main target alternative and returns it.
#
rule main-target-alternative ( target )
{
    local ptarget = [ $(target).project ] ;
    $(ptarget).add-alternative $(target) ;
    return $(target) ;
}


# Creates a metatarget with the specified properties, using 'klass' as the
# class. The 'name', 'sources', 'requirements', 'default-build' and
# 'usage-requirements' are assumed to be in the form specified by the user in
# the Jamfile corresponding to 'project'.
#
rule create-metatarget ( klass : project : name : sources * : requirements * :
    default-build * : usage-requirements * )
{
    return [ targets.main-target-alternative [ new $(klass) $(name) : $(project)
        : [ targets.main-target-sources $(sources) : $(name) ]
        : [ targets.main-target-requirements $(requirements) : $(project) ]
        : [ targets.main-target-default-build $(default-build) : $(project) ]
        : [ targets.main-target-usage-requirements $(usage-requirements) :
            $(project) ] ] ] ;
}


# Creates a typed-target with the specified properties. The 'name', 'sources',
# 'requirements', 'default-build' and 'usage-requirements' are assumed to be in
# the form specified by the user in the Jamfile corresponding to 'project'.
#
rule create-typed-target ( type : project : name : sources * : requirements * :
    default-build * : usage-requirements * )
{
    return [ targets.main-target-alternative [ new typed-target $(name) :
        $(project) : $(type)
        : [ targets.main-target-sources $(sources) : $(name) ]
        : [ targets.main-target-requirements $(requirements) : $(project) ]
        : [ targets.main-target-default-build $(default-build) : $(project) ]
        : [ targets.main-target-usage-requirements $(usage-requirements) :
            $(project) ] ] ] ;
}
